If the volume of parallelopiped formed by the vectors $\hat{\mathrm{i}}+\lambda \hat{\mathrm{j}}+\hat{\mathrm{k}},\hat{\mathrm{j}}+\lambda \hat{\mathrm{k}}\mathrm{and}\lambda \hat{\mathrm{i}}+\hat{\mathrm{k}}$ is minimum, then $\lambda$ is equal to